Origin automatically sets itself to Off-line mode when it doesn't have any internet signal.
I've been using the setup.exe as you said, but nothing really happens. When clicked on, it opens the "ME3/Origin Install Now" pop-up, the disc spins for a bit but nothing happens.
Do you have a virus scanner that blocks access to the auto-run function or to the drive entirely?
I installed the previous two games without any problems, and checking my anti-virus program, it didn't block any programs from running when I tried to install the disc offline, nor when online.
July - last edited July
I just tested with my own disk and it was not even necessary to put Origin in offline mode.
I insert the disk while Origin was closed and opened the drive. Then I executed the "Setup.exe"
It opened Origin and started to install the game from disk: (Wird Kopiert = Is copied)
Compare with the download: (Download Läuft = Downloaded)
Did you tried it this way?